OK that's the first obvious check out of the way, the next one would be to make sure that the CPU fan is working OK and that the CPU heatsink has not come off ( unlikely I know but you still need to check ).
What antivirus do you use? Is it up to date? Have you checked for the Blaster worm virus? You might also check for the rpc exploit (if you are not fully patched) (unlikely to be the problem but another possible)
I would suggest (asuming no virus) you get hold of some more ram and try it. It is very cheap these days and if you got 256 at least you could double your ram if that is not the fault. ( I would heavily supect the ram)